1. Field of the Invention
The present invention is directed to method and apparatus for curing a coating of an optical fiber, or optical fiber ribbon, using one or more lasers. More particularly, the invention is directed to a method and apparatus where a plurality of laser beams are irradiated on the fiber from different angles to provide a more uniform cure of the fiber coating.
2. Related Art
In the manufacture of an optical fiber, a glass preform rod which generally is manufactured in a separate process is moved into a furnace at a controlled rate with the rod positioned vertically. The furnace softens the preform so that the optical fiber is drawn from the molten end of the preform rod by a capstan located at the base of a draw tower.
Because the surface of the optical fiber is very susceptible to damage caused by abrasion, a coating must be applied to the optical fiber, after it is drawn, but before it comes into contact with any surface. Once the liquid coating is applied, the coating material must become solidified rapidly before the optical fiber reaches a capstan. This is generally accomplished by photocuring.
While a single coating is applied in some instances, typically, two coatings are applied including a primary coating and a secondary coating. The primary coating is applied directly to the glass fiber and, when cured, forms a soft, compliant material for cushioning the glass fiber so as to protect it by relieving stresses created when the fiber is bent, cabled or spooled. The secondary coating is applied over the primary coating and functions as a protective outer layer for preventing damage to the glass fiber during manufacturing and use.
After the coating material or materials have been applied to the moving optical fibers, the coating material or materials are cured, typically by exposure to ultraviolet radiation. In some coating systems, a primary coating material is applied and cured by subjecting it to ultraviolet energy prior to the application of the secondary coating material. An important consideration in the manufacturing optical fibers is to minimize the amount of heat in the fiber during the coating process. For example, it has been discovered that the modulus of the coating material on the optical fiber is a function of the temperature at which the curing of the coating material occurred. An undesired temperature can occur if an excessive amount of infrared radiation reaches the coating material which will have an adverse impact on the resulting modulus of the coating material.
Another consideration is that the fiber is already very hot itself as a result of the drawing process. If the fiber is too hot as it enters the fluid in the coating die a thermal boundary layer will form creating coating instability and/or allowing the coating to not coat the fiber at all. Adding heat from the lamps reduces the rate of cooling of the fiber thus increasing the chances of this occurringxe2x80x94i.e. the cooler the environment around the fiber, the faster the fiber will cool. In the ribbon process, this pre-existing heat is not a problem because the fibers are at room temperature going into the coating die. However, there is still the potential for creating variations in finished material properties, such as modulus, and the problem of wasted energy used to generate useless wavelengths of radiation, as discussed below.
UV curing relies on both the intensity and wavelength of the UV radiation, as well as on the corresponding absorption of the photoinitiators and photosensitizers in the UV-curable material. Conventional irradiators use a lamp that emits a wide variety of wavelengths with peak irradiations at one or more wavelengths or a small band of wavelengths. A certain level of inefficiency is inherent in such lamps simply because the majority of the output power is applied over a broad band of wavelengths that may not be helpful in the curing process. Additionally, such lamps typically require a high volume of air be pulled through the lamp configuration in order to keep it cool. This air is flowing in the area between the lamps bulb and the optical fiber or optical fiber ribbon, hereafter referred to as the substrate (i.e. substrate is either on optical fiber or an optical fiber ribbon). The oxygen in this air will react with a portion of the UV radiation emitted and form ozone, which must then be dealt with as a safety concern. The UV radiation that is consumed in this reaction with oxygen never reaches the substrate and is therefore lost as an inefficiency. A laser system, not generating the radiative heat of the conventional lamps, does not require this cooling airflow in the region between the laser and the substrate. Therefore, this region may be filled with an inert gas such as nitrogen so that there is no oxygen in this region to consume any UV radiation. With this in mind, not only does the laser not produce wasted radiation, but also it can be used in such a way as to deliver the radiation produce in a more efficient manner.
U.S. Pat. No. 4,812,150 discloses using a laser to cure coatings of an optical fiber. However, in this patent, a single laser beam impinges on one side of the optical fiber such that the heating of the coating is uneven. Curing an optical fiber, and especially an optical fiber ribbon, in this manner will cause the substrate to bow since the curing on one side is greater than the curing on the other side. Further, excess heat may be generated due to the localized focus of the laser beam resulting in a coating with an undesirable modulus. If a UV-curable laser is used in the arrangement described in this prior art (rather than the heat generating lasers described), there will be little or no heating due to the localization of the laser""s output wavelength into a relatively narrow band. However, the potential for uneven cure, and thus bowing, is equally true with any wavelength laser if the laser is primarily focused on one side of the optical fiber or optical fiber ribbon.
It is an object of the present invention to overcome the drawbacks of the above related art. More particularly, it is an object of the present invention to provide a system and a method for curing a coating applied to an optical fiber substrate in which the temperature of the coating during curing is maintained in a preferred range.
It is a further object of the invention to provide a system and a method of curing a coating of an optical fiber substrate where the wavelength of the beams used to cure the coating is controlled to optimize polymerization efficiency and minimize wasted energy (i.e. energy used to create wavelengths that do not activate the photoinitiators/sensitizers and energy used to generate the UV-radiation that is absorbed by oxygen in conventional lamp systems).
The present invention achieves the above and other objects and advantages by providing a laser curing device which emits a plurality of laser beams, having a predetermined wavelength or wavelength range, that impinge on the fiber at differing angles to provide a uniform cure.
According to one aspect of the invention, the system includes at least one laser for outputting a laser beam; a splitter for splitting the laser beam into a plurality of output beams; and a reflector device for reflecting the output beams such that the output beams are irradiated on the coating of the optical fiber from different angles. The reflector device comprises a plurality of mirrors or, alternatively, a housing, which substantially surrounds the optical fiber where at least a portion of an inner surface of the housing is reflective.
The splitter includes a beam splitter or any other equivalent means for splitting a beam. The laser beam has a preferred wavelength in the range of 250-450 nm, however, the optimal wavelength will be dependent on the specific photoinitiators/sensitizers used in the material being cured. For instance, the shorter wavelengths are more energetic and will typically provide a faster cure rate, while the longer wavelengths tend to more adequately penetrate through contaminants, such as pigments, and are therefore generally more suitable for pigmented materials.
According to another aspect of the invention, the. system includes a second laser for outputting a second laser beam which impinges on the fiber substrate, where the wavelength range of the second laser is in range that may be different from the wavelength range of the first laser to cure at different depths, or may be the same to provide additional energy and thus reduce the required exposure time (increasing production speed).
According to yet another aspect of the invention, a method of curing the coating of an optical fiber substrate includes the step of directing a plurality of beams emitted by at least one laser toward the substrate such that the beams impinge on the substrate at different angles.